Generally, an automatic transmission for a vehicle includes a Transmission Control Unit (TCU) that automatically regulates a gear ratio according to various driving conditions such as vehicle speed and engine load. The TCU regulates the output shaft speed of a planetary gear set by controlling the operation of the clutches and brakes within the gear train. To regulate the output shaft speed, the TCU receives signals representative of vehicle driving parameters and determines a solenoid valve duty control signal according to a predetermined program. The solenoid valve regulates hydraulic pressure supplied to the clutches and brakes and is operated according to the solenoid valve duty control signal.
The TCU determines whether the output shaft speed sensor has failed or not based on the output shaft speed signal. For example, the TCU concludes the output shaft speed sensor has failed if a difference between the output shaft speed signal and the vehicle speed signal is above 30% of the output shaft speed signal. If the output shaft speed sensor has failed, the TCU does not allow the transmission to shift to a target gear, holds the gear in second gear or third gear, and displays an engine inspection lamp instructing the driver to seek maintenance for the failed part. But it is impossible to diagnose the output shaft speed sensor if the engine speed sensor is removed. Furthermore, it is possible to incorrectly determine the output shaft speed sensor has failed when the engine speed sensor has interference.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.